Present commercial operations for hermetically sealing open-ended packaging containers, especially food containers, generally employ either (1) a conventional metallic end, with or without an easy-opening feature, which is double-seamed to a flange on the container, or (2) a film or foil lidding structure, which is attached by adhesive, thermal or other bonding means to a flange on the open end of the container.
With the increasing demand for convenience packaging and ready-to-eat food products, a growing need exists for shelf-stable microwavable food packages. A significant number of these are semi-rigid plastic packaging containers. Food packages for these applications are generally required to undergo a retort/sterilization process to insure that the product is sterile and safe for consumption. Because of the high temperatures and internal pressures associated with the retort process, it is an extremely critical part of the packaging operation to provide a packaging container and closure therefor which have great strength and integrity.
Conventional double-seamed metal ends or closures employed to seal these containers provide seal integrity throughout the retort process, and can provide a convenient easy-open feature for the consumer. The metal end, however, is not entirely appropriate for a package designed to be reheated in a microwave oven, due to arcing and localized heating which can occur during heating. The metal end is also generally not suitable for very wide containers, such as bowls or trays, for a variety of reasons, including the high cost of such an end, and the aesthetically negative appearance of such a package. Additionally, the metal end cannot be used at all in those instances where the heating method of retort comes from microwave energy, due to the very large metallic surface area which would reflect energy and defeat the ability of the microwave energy to heat the package contents. Ready-to-eat packages which employ a metal end often have a sharp metal edge exposed after removal of the easy-open lid, which presents a possible consumer safety hazard.
Film or foil lidding systems as applied to retortable packages are the most sensitive materials used in thermal processing. The adhesive or heat-sealed bond between the body of the packaging container and the lidding material is very fragile during the heat processing period. The bond strength can be reduced by as much as 80% at elevated temperatures. Pressures inside the sealed container can rise dramatically during heating, caused by expansion of the product and gases, which place further demands on the sealing area. An additional challenge to the processor is that many of the film/foil lidding systems are designed to have peelable seals to allow for easy opening. An improved type of flexible lidding systems is disclosed, for example, in U.S. Pat. Nos. 4,801,041; 4,858,780; 4,905,838 and 4,913,307, assigned to Idemitsu Petrochemical Co., Ltd. and commercialized as "MAGICTOP" and which overcomes some of the problems with prior flexible lidding systems. However, all of these flexible lidding systems are attached by bonding to the open end of the container and most processors who employ these types of flexible lidding system must perform extensive testing of seal integrity, including 100% inspection, and require very tight controls on retort conditions to insure against seal failure and/or spoilage of contents.